METHOD OF MAINTAINING HEALTH OF A FLOW BATTERY
BACKGROUND
[0001] Flow batteries, also known as redox flow batteries or redox flow cells, are
designed to convert electrical energy into chemical energy that can be stored and later
released when there is demand. As an example, a flow battery may be used with a renewable
energy system, such as a wind-powered system, to store energy that exceeds consumer
demand and later release that energy when there is greater demand.
[0002] A typical flow battery includes a redox flow cell that has a negative
electrode and a positive electrode separated by an electrolyte layer, which may include a
separator, such as an ion-exchange membrane. A negative fluid electrolyte (sometimes
referred to as the anolyte) is delivered to the negative electrode and a positive fluid
electrolyte (sometimes referred to as the catholyte) is delivered to the positive electrode to
drive electrochemically reversible redox reactions. Upon charging, the electrical energy
supplied causes a chemical reduction reaction in one electrolyte and an oxidation reaction in
the other electrolyte. The separator prevents the electrolytes from freely and rapidly mixing
but permits selected ions to pass through to complete the redox reactions. Upon discharge, the
chemical energy contained in the liquid electrolytes is released in the reverse reactions and
electrical energy can be drawn from the electrodes. Flow batteries are distinguished from
other electrochemical devices by, inter alia, the use of externally-supplied, fluid electrolyte
solutions that include reactants that participate in reversible electrochemical reactions.
SUMMARY
[0003] Disclosed is a method of maintaining health of a flow battery that includes
determining an average oxidation state of a common electrochemically active elemental
specie in first and second fluid electrolytes on, respectively, a positive side and a negative
side of an electrochemical cell of a flow battery, and adjusting the average oxidation state in
response to the average oxidation state deviating from a predefined range of average
oxidation state values.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] The various features and advantages of the present disclosure will become
apparent to those skilled in the art from the following detailed description. The drawings that
accompany the detailed description can be briefly described as follows.
[0005] Figure 1 illustrates an example flow battery.
[0006] Figure 2 illustrates an example method of maintaining health of a flow
battery.
DETAILED DESCRIPTION
[0007] Figure 1 schematically shows portions of an example flow battery 20 for
selectively storing and discharging electrical energy. As an example, the flow battery 20 can
be used to convert electrical energy generated in a renewable energy system to chemical
energy that is stored until a later time when there is greater demand, at which time the flow
battery 20 then converts the chemical energy back into electrical energy. The flow battery 20
can supply the electric energy to an electric grid, for example.
[0008] The flow battery 20 includes a fluid electrolyte 22 that has an
electrochemically active specie 24 that functions in a redox pair with regard to an additional
fluid electrolyte 26 that has an electrochemically active specie 28. The electrochemically
active species 24/28 are common and are based on vanadium, iron, or chromium, for
example. That is, in one example, the electrochemically active species 24/28 are differing
oxidation or valence states of vanadium, and in another example the electrochemically active
species 24/28 are differing oxidation or valence states of iron, and in another example the
electrochemically active species 24/28 are differing oxidation or valence states of chromium.
The fluid electrolytes 22/26 are liquid solutions that include the electrochemically active
species 24/28. The first fluid electrolyte 22 (e.g., the negative electrolyte) and the second
fluid electrolyte 26 (e.g., the positive electrolyte) are contained in a supply/storage system 30
that includes first and second vessels 32/34 and pumps 35.
[0009] The fluid electrolytes 22/26 are delivered from the first and second vessels
32/34, using the pumps 35, to at least one cell 36 of the flow battery 20 through respective
feed lines 38. The fluid electrolytes 22/26 are returned from the cell 36 to the vessels 32/34
via return lines 40. The feed lines 38 and the return lines 40 connect the vessels 32/34 with
first and second electrodes 42/44 of the cell. Multiple cells 36 can be provided as a stack.
[0010] The cell or cells 36 each include the first electrode 42, the second electrode
44 spaced apart from the first electrode 42, and an electrolyte separator layer 46 arranged
between the first electrode 42 and the second electrode 44. For example, the electrodes 42/44
are porous carbon structures, such as carbon paper or felt. In general, the cell or cells 36 can
include bipolar plates, manifolds and the like for delivering the fluid electrolytes 22/26
through flow field channels to the electrodes 42/44. The bipolar plates can be carbon plates,
for example. It is to be understood, however, that other configurations can be used. For
example, the cell or cells 36 can alternatively be configured for flow-through operation where
the fluid electrolytes 22/26 are pumped directly into the electrodes 42/44 without the use of
flow field channels.
[0011] The electrolyte separator layer 46 can be an ionic-exchange membrane, an
inert micro-porous polymer membrane or an electrically insulating microporous matrix of a
material, such as silicon carbide (SiC), that prevents the fluid electrolytes 22/26 from freely
and rapidly mixing but permits selected ions to pass through to complete the redox reactions
while electrically isolating the electrodes 42/44. In this regard, the fluid electrolytes 22/26 are
generally isolated from each other during normal operation, such as in charge, discharge and
shutdown states.
[0012] The fluid electrolytes 22/26 are delivered to the cell 36 to either convert
electrical energy into chemical energy or, in the reverse reaction, convert chemical energy
into electrical energy that can be discharged. The electrical energy is transmitted to and from
the cell 36 through an electric circuit 48 that is electrically coupled with the electrodes 42/44.
[0013] The flow battery 20 can experience self-discharge reactions. For example,
self-discharge can occur when electrochemically active species 24/28 in the fluid electrolytes
22/26 permeate through the electrolyte separator layer 46. Permeation is not desirable but
can occur since the electrolyte separator layer 46 is not a perfect barrier to the transport of the
fluid electrolytes 22/26. Additionally, the electrochemically active species 24/28 can diffuse
through the electrolyte separator layer 46 due to the concentration gradient of the different
species that exists across the separator layer 46. Furthermore, as is the case in any battery,
undesirable side reactions can also occur, which can result in the evolution of hydrogen or
oxygen, especially in aqueous electrolytes, and these adverse side reactions on one electrode
can also result in the oxidation or reduction of the electrochemically active species 24/28 on
the opposite electrode. Over time, these reactions can cause an imbalance between the
electrochemically active species 24/28 that debits the performance and maximum energy
capacity of the flow battery 20. Moreover, such an imbalance can indicate poor health of the
flow battery 20, as well as a need to rebalance the electrochemically active species 24/28.
[0014] Figure 2 shows an example method 50 of maintaining health of the flow
battery 20. For example, the health of the flow battery 20 can be represented by an average
oxidation state of the electrochemically active species 24/28. The average oxidation state
provides a convenient, single value that is a good indicator of the health of the fluid
electrolytes 22/26 and, thus, the flow battery 20.
[0015] As shown, the method 50 generally includes a determination step 52 and
an adjustment step 54, although it is to be understood that the steps 52/54 can be utilized in
combination with other steps or procedures as may be appropriate. The determination step 52
includes determining an average oxidation state of the electrochemically active species 24/28
in the fluid electrolytes 22/26. The adjustment step 54 includes then adjusting the average
oxidation state in response to the average oxidation state deviating from a predefined range of
average oxidation state values.
[0016] The predefined average oxidation state value can be a single value or a
value range. The average oxidation state of the electrochemically active species 24/28 is then
adjusted if it deviates from the single value or outside of the value range. For instance, the
value range can be based upon the type of common electrochemically active species 24/28. In
a further example, the value range can be +/- 10% of the average oxidation state when the
common electrochemically active species 24/28 are fully balanced. For vanadium, the fully
balanced average oxidation state is +3.5 based upon the use of V +/V + and V4+/V5+ (which
can also be denoted as V(ii)/V(iii) and V(iv)/V(v), although the valences of the vanadium
species with oxidation states of 4 and 5 are not necessarily 4+ and 5+) as the
electrochemically active species 24/28. For example, if the electrolyte solution is aqueous
sulfuric acid, then the V(iv)/V(v) species will be present as V0 + and V0 2
+, respectively.
[0017] The average oxidation state of the electrochemically active species 24/28
can be determined either directly or indirectly. If determined directly, the average oxidation
state can be determined from measurements of molar concentrations of different valence
states of the electrochemically active species 24/28. In one example based on vanadium as
the common electrochemically active species 24/28, the valence states include V +/V + and
V4+/V5+. In one further example, the molar concentrations of the different valence states of
the electrochemically active species 24/28 can be obtained using an optical detector device
that collects molar concentration measurements of the electrochemically active species 24/28
in one or both of the vessels 32/34 using optical measurements. For example, the optical
detector device utilizes light to detect molar concentration. Once the molar concentrations are
obtained, the average oxidation state can be determined as a function of the molar
concentrations divided by a total molar amount of the electrochemically active species 24/28.
In one example based upon vanadium, the average valence state can be determined according
to equation I :
Equation I :
- = (2 2+ + 3 3+ )V_ + (4c4+ + 5 + )V+
N
[0018] In Equation I, v represents the average oxidation state, c2+ represents the
concentration of V(ii), C3+ represents the concentration of V(iii), c4+ represents the
concentration of V(iv), c + represents the concentration of V(v), and Nt,o represents the total
moles of the electrochemically active species 24/28. Similarly, the average oxidation state
can be determined for an electrochemically active specie based on iron or chromium or other
common specie.
[0019] The average oxidation state can alternatively be determined by directly
measuring the concentrations of the different valence states using a titration technique,
although the titration technique is not a real-time measurement and may, therefore,
potentially introduce a time lag.
[0020] Alternatively or in addition to determining the average oxidation state
directly, the average oxidation state can be determined indirectly from other properties of the
fluid electrolytes 22/26. For example, measurements can be collected of conductivity,
density, viscosity or combinations thereof of the fluid electrolytes 22/26. The conductivity,
density and/or viscosity can be correlated experimentally to the average oxidation state of the
electrochemically active species 24/28. Thus, these indirect measurements can be used to
determine the average oxidation state or alternatively to verify direct measurements.
[0021] Once the average oxidation state is known, the average oxidation state can
be adjusted if it deviates from the predefined average oxidation state value at the adjustment
step 54. For example, if the average oxidation state is outside of the predefined average
oxidation state value or value range, the average oxidation state is adjusted back to an
acceptable level, for example.
[0022] Any of several techniques can be used to adjust the average oxidation
state, depending upon the determined average oxidation state in the method 50. In one
example, if the average oxidation state is too high, a reducing agent can be added to the
second fluid electrolyte 26 on the positive side of the cell 36 to reduce the valence state of the
electrochemically active specie 28 in the second fluid electrolyte 26. In some examples, the
reducing agent is an acid, such as oxalic acid or formic acid. Additionally, or alternatively, an
alcohol can be used as the reducing agent. In one example based upon oxalic acid, the oxalic
acid reacts with the electrochemically active specie 28 to thus reduce the average valence
state. A byproduct of the reaction is the generation of carbon dioxide, which is not harmful to
the flow battery 20. Thus, the use of oxalic acid additionally provides the benefit of avoiding
the generation of toxic chemicals or chemicals that would otherwise debit the performance of
the flow battery 20.
[0023] If the average oxidation state is too low, an oxidizing agent can be added
to the first fluid electrolyte 22 on the negative side of the cell 36 to increase valence state of
the electrochemically active specie 24. For example, the oxidizing agent includes oxygen gas,
such as air, and/or hydrogen peroxide. In one example based upon air, the air can be
introduced and exposed to the first fluid electrolyte 22 simply by opening the vessel 32 to
permit entry of air. In another example, the first fluid electrolyte 22 can be agitated within the
vessel 32 or pumped using the pump 35 to increase exposure. Additionally or alternatively,
air or oxygen gas can be bubbled through the first fluid electrolyte 22 in the vessel 32 to
further increase exposure to the first fluid electrolyte 22.
[0024] In further examples, deviation of the average oxidation state from the
predefined average oxidation state value or value range is indicative of poor health of the
flow battery 20. Thus, there may be a need to adjust how the flow battery 20 is operated in
order to mitigate or reduce change in the average oxidation state in the future. For example,
the side reactions that tend to cause a shift in the average oxidation state can be caused by
high over-potentials or exceeding maximum voltages in the flow battery 20 and thus promote
these adverse side reactions. To mitigate these side reactions, the charge capacity of the flow
battery 20 can be reduced in response to the average oxidation state deviating from the
predefined average oxidation state value or value range. By reducing such charge capacity,
lower over-potentials can be obtained, which can facilitate avoidance of future deviations of
the average oxidation state from the predefined average oxidation state value or value range.
[0025] Additionally, if the flow battery 20 is known to have a tendency to creep
towards an average oxidation state that is too high or too low, the average oxidation state can
be initially set to a correspondingly lower or higher level in order to extend a period of time
before having to adjust the average oxidation state. For instance, the average oxidation state
can initially be set to a relatively high level in response to an expectation that the average
oxidation state will creep to a lower value that eventually falls below the predefined average
oxidation state value or value range.
[0026] Although a combination of features is shown in the illustrated examples,
not all of them need to be combined to realize the benefits of various embodiments of this
disclosure. In other words, a system designed according to an embodiment of this disclosure
will not necessarily include all of the features shown in any one of the Figures or all of the
portions schematically shown in the Figures. Moreover, selected features of one example
embodiment may be combined with selected features of other example embodiments.
[0027] The preceding description is exemplary rather than limiting in nature.
Variations and modifications to the disclosed examples may become apparent to those skilled
in the art that do not necessarily depart from the essence of this disclosure. The scope of
legal protection given to this disclosure can only be determined by studying the following
claims.

CLAIMS
What is claimed is:
1. A method of maintaining health of a flow battery, the method comprising:
(a) determining an average oxidation state of a common electrochemically active
elemental specie in first and second fluid electrolytes on, respectively, a positive side and
a negative side of an electrochemical cell of a flow battery; and
(b) adjusting the average oxidation state in response to the average oxidation state
deviating from a predefined average oxidation state value.
2. The method as recited in claim 1, wherein the common electrochemically active
elemental specie is selected from the group consisting of vanadium, iron, and chromium.
3. The method as recited in claim 1, wherein said step (a) includes directly determining
the average oxidation state from concentrations of different valence states of the common
electrochemically active elemental specie in first and second fluid electrolytes.
4. The method as recited in claim 3, wherein said step (a) includes determining the
average oxidation state as a function of molar concentrations of the different valence states
divided by a total molar amount of the common electrochemically active elemental specie.
5. The method as recited in claim 1, wherein said step (b) includes adding a reducing
agent to the second fluid electrolyte on the positive side to reduce a valence state of the
common electrochemically active elemental specie in the second fluid electrolyte.
6. The method as recited in claim 5, wherein the reducing agent includes an acid.
7. The method as recited in claim 5, wherein the reducing agent includes oxalic acid.
8. The method as recited in claim 5, wherein the reducing agent includes formic acid.
9. The method as recited in claim 5, wherein the reducing agent includes an alcohol.
10. The method as recited in claim 1, wherein said step (b) includes adding an oxidizing
agent to the first fluid electrolyte on the negative side to increase a valence state of the
common electrochemically active elemental specie in the first fluid electrolyte.
11. The method as recited in claim 10, wherein the oxidizing agent includes oxygen gas.
12. The method as recited in claim 10, wherein the oxidizing agent includes air.
13. The method as recited in claim 10, wherein the oxidizing agent includes hydrogen
peroxide.
14. The method as recited in claim 1, wherein the predefined average oxidation state
value is an average oxidation state range.
15. The method as recited in claim 1, wherein said step (a) includes collecting
measurements representing concentrations of different valence states of the common
electrochemically active elemental specie in first and second fluid electrolytes.
16. The method as recited in claim 15, wherein the measurements include concentration
measurements.
17. The method as recited in claim 15, wherein the measurements are selected from the
group consisting of optical measurements, conductivity measurements, density
measurements, viscosity measurements and combinations thereof.
18. The method as recited in claim 1, further comprising reducing a charge capacity of the
flow battery in response to the average oxidation state deviating from the predefined average
oxidation state value.
19. The method as recited in claim 1, wherein the flow battery comprises:
the electrochemical cell including a first electrode, a second electrode spaced
apart from the first electrode and an electrolyte separator layer arranged between the first
electrode and the second electrode, and
a supply/storage system external of the electrochemical cell, the supply/storage
system including first and second vessels fluidly connected with the electrochemical cell.
20. The method as recited in claim 1, further comprising adjusting the average oxidation
state to be either higher or lower than the predefined average oxidation state value in
response to an expectation that the average oxidation state will creep toward the predefined
average oxidation state value.